Oscillators are widely used in various microelectronic systems for such purposes as providing clock signals. In some applications, oscillators are used to produce steady stable clock signals which are relatively insensitive to temperature variations. For example, some oscillators in motion control encoders are required to operate across a wide temperature range, such as between −40 to 125 degrees Celsius.
To achieve accurate and stable clock signal performance over such a wide temperature range, designers typically employ external components such as crystals and inductors. These external solutions, however, place high cost and size burdens on the resulting circuit. Oscillators with external crystals and LC circuits are also difficult to integrate into monolithic circuits, and are usually characterized by narrow frequency bandwidths.
Relaxation oscillators, however, can be integrated in monolithic circuitry at relatively low cost and with small size. The frequency of a relaxation oscillator can be programmed, and thus can operate over a wide frequency band. Most relaxation oscillator circuits are sensitive to temperature variations, however, if temperature compensation circuitry is not employed.
In the current state of the art, temperature compensation techniques are employed to reduce the temperature coefficient of a relaxation oscillator. Basically, the frequency of the relaxation oscillator is made proportional to a charging current and inversely proportional to a threshold voltage.
In a first example of a prior art temperature-compensated relaxation oscillator circuit, and as described in further detail in U.S. Pat. No. 6,720,836 to Xijian Lin entitled “CMOS relaxation oscillator circuit with improved speed and reduced process temperature variations,” the frequency of a relaxation oscillator is expressed as F=ISINK/(2·C1·VCLMP). As a result, the frequency is proportional to the charging current, ISINK, and inversely proportional to the value of the timing capacitor C1 and the threshold voltage VCLMP. The threshold voltage VCLMP is largely insensitive to the temperature variation. VCLMP can be expressed as VCLMP=k·Vref, where k is a constant and Vref is a bandgap voltage. Because ISINK must be insensitive to temperature to make the frequency of relaxation oscillator independent of temperature, the capacitor, C1, must have a low temperature coefficient. A precise low temperature coefficient resistor, such as an off-chip resistor Rext, must therefore to be employed when using this temperature compensation technique, which increases circuit cost and size.
In a second example of a prior art temperature-compensated relaxation oscillator circuit, and as described in further detail in U.S. Pat. No. 6,157,270 to Vincent Wing Sing Tso entitled “Programmable highly temperature and supply independent oscillator,” the frequency of a relaxation oscillator is proportional to the charging current and the threshold voltage Vth. The basic concept of this approach is to generate charging currents and threshold voltages having temperature-dependent parameters that substantially cancel one another to yield a temperature-independent output signal of constant frequency. While this temperature compensation technique reduces the temperature coefficient of the relaxation oscillator to around 294 ppm/° C. under typical operating conditions, this approach does not take into account the temperature coefficient of the resistor of the relaxation oscillator, with the result that temperature-induced variations in the output signal of the relaxation oscillator will occur unless a high precision off-chip resistor is used, which increases circuit cost and size.
In a third example of a prior art temperature-compensated relaxation oscillator circuit, and as described in further detail in U.S. Pat. No. 6,356,161 to James B. Nolan et al. entitled “Calibration techniques for a precision relaxation oscillator integrated circuit with temperature compensation,” an expensive and space-consuming low temperature coefficient external off-chip resistor Rext is also employed to produce a more stable clock output signal of constant frequency, which is largely independent of temperature.
In a fourth example of a prior art temperature-compensated relaxation oscillator circuit, and as described in further detail in U.S. Pat. No. 5,699,024 to Gregory Jon Manlove et al. entitled “Accurate integrated oscillator circuit,” there is provided an oscillator circuit having an acceptable degree of temperature independence. The temperature compensation circuit of Manlove et al. relies on the temperature behavior of a bipolar transistor, and thus requires the use of a bi-CMOS manufacturing process. This special requirement increases circuit cost.
What is needed is temperature compensation circuitry that may be used in conjunction with a relaxation oscillator to provide a low cost, small size, substantially temperature-insensitive, wide-frequency-band, clock circuit.